The Biology of Northern Shrimp (Pandalus borealis) on the Flemish Cap

This study reports on various biological features in the life history of northern shrimp (Pandalus borealis) on Flemish Cap. Northern shrimp on Flemish Cap are males for approximately their first three years. Then usually half change sex at age four years, and the rest at age five years, to become females from then on. Small shrimp are generally found at lesser depth than the larger ones so the size of shrimp increases with depth, especially that of males. Larger females seem to move to shallower waters in March when hatching starts. The size at sex change (L50) and the maximum length (Lmax) are studied within and between seasons. Both L50 and Lmax have decreased in recent years. L50 was 22.5 mm CL on average in the seasons 1996/1997–2003/2004, but has decreased from 23.2 mm CL in 1997/1998 to 22.4 mm CL in 2003/2004 (Icelandic commercial data) and from 24.3 mm CL in 1994 to 20.9 mm CL in 2003 (EU survey data). Also Lmax has decreased from 29.8 mm CL in 1996/1997 to 28.0 mm CL in 2003/2004 (Icelandic commercial data). In spite of this, there is an invariant re-lationship between L50 and Lmax, where L50 is about 80 % of the average Lmax of northern shrimp on Flemish cap. This supports the theory of Charnov and Skúladóttir (2000) on the invariant relationship between L50 and Lmax in sex changing organisms. The same is found in Icelandic offshore waters an